H.E.S.S.

High Energy Stereoscopic System

More about HESS J1708-410

May 2010

Very high energy
gamma-ray image of HESS J1708-410.
The dashed line indicates the
galactic equator.

Roughly half of the gamma-ray
sources discovered by H.E.S.S. lack
definitive counterparts at other wavelengths, and a few lack any
plausible counterpart. HESS J1708-410 (Aharonian
et al. 2008, top image) is one
of
those few VHE gamma-ray sources for which no plausible explanation has
yet been put forward (Van
Etten et al. 2009). HESS
J1708-410 was discovered during the H.E.S.S. survey of the inner Galaxy
(Aharonian
et al. 2006); subsequent
observations improved the
statistical significance to 11 sigma. The compact source is only
marginally extended on the scale of the H.E.S.S. angular resolution,
consistent with a slightly elongated shape of approximately 0.08 degr.
size. The spectrum of gamma rays extends from 0.5 TeV to beyond 30 TeV,
with an energy flux of 8x10-12
ergs/cm2s.
A multiwavelength view of the region (Fig. 1)
does not reveal counterparts; the supernova remnant G345.7-00.2 is well
separated from the gamma-ray source, as are radio hotspots and ROSAT
X-ray sources.

The discussion of the nature of
HESS J1708-410 was reopened by Van
Etten et al. (2009), based on a
new XMM-Newton 27 ks exposure of the region and on archival radio
continuum and CO data, searching for either extended X-ray emission
indicative of a population of high-energy electrons, or molecular
clouds which might serve as a target for accelerated cosmic rays, and
possibly host the accelerator. The new X-ray data show four X-ray point
sources in the source region, and three additional sources at the
periphery of the gamma-ray source. None of the X-ray sources indicates
any sign of extended emission, as one would require for a plausible
counterpart of the extended gamma-ray source. For diffuse X-ray flux
from the region of HESS J1708-410, a flux limit
of 3x10-13 erg/cm2s is
given for the 2-4 keV range, well below the energy flux observed in
gamma rays. The lack of diffuse X-ray emission is easily
explained if HESS J1708-410 is an accelerator of cosmic ray nuclei,
illuminating a nearby target. Analysis of CO data of Dame et al. (2001)
in a 0.15 x 0.15 degr. pixel covering the gamma-ray sources indeed
revealed a molecular complex at about 3 kpc distance (Fig.
2).
To account for the observed gamma-ray flux assuming a modest target
density of 10/cm3
in the cloud, a cosmic-ray energy input around 1050
ergs would be required, and could be provided by a typical supernova
explosion. However, also the scenario of an electron accelerator cannot
conclusively be ruled out. If the electron spectrum breaks off at high
energy, either due to cooling losses and intrisic properties of the
acceleration process, one can construct scenarios where the very high
energy gamma ray spectrum is well described and the limit on the diffuse
X-ray flux is obeyed (Fig.
3); in such scenarios, the synchrotron luminosity of
the source is peaked well below the XMM range, and remains undetected.

Hence, the discussion about the
nature of this gamma-ray source remains open; deeper radio observations
might provide further clues.

Fig. 1:
Same field as
the top image, showing as black lines the gamma-ray source contours, in
green ROSAT X-ray source contours, overlayed on the Molonglo radio
image in grey-scale. Also plotted are the positions of SNR G345.7-0.2,
the ROSAT source 1RXS J171011.5-405356 (marked "X") and the low mass X-ray
binary 4U 1708-40.

Fig. 2:
CO velocity profile in the vicinity of HESS J1708-410, showing a single
major molecular complex along the line of sight, at a probable distance
of 3 kpc. The scale on top shows the relation between velocity and
distance, according to the galactic rotation curve. From Van
Etten et al. 2009.